Opencv shows images totally white?

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Hi guys, can any of you tell me if there's a bug in the convertTo function?

With reference to this question, I'm running that code and I'm getting the pictures shown.

The code I'm running is this one:

int testImageAndGradients(int argc, char **argv) {
    if (argc != 2) {
        std::cout << "Need filename..." << std::endl;
        return 1;
    }

    Mat im = imread(argv[1], CV_LOAD_IMAGE_GRAYSCALE);

    Mat imfloat, dx, dy;
    im.convertTo(imfloat, CV_32FC1);
    Mat u(imfloat.size(), imfloat.type());

    int & X = im.cols;
    int & Y = im.rows;

    Sobel(imfloat, dx, CV_32F, 1, 0, 3);
    Sobel(imfloat, dy, CV_32F, 0, 1, 3);

    Mat tmp, output(Size(3*X,Y),CV_8UC1);

    double minval, maxval;

    minMaxLoc(imfloat, &minval, &maxval);
    imfloat.convertTo(tmp, CV_8UC1, 255 / (maxval - minval), -minval);
    tmp.copyTo(output(Rect(0, 0, X, Y)));

    minMaxLoc(dx, &minval, &maxval);
    dx.convertTo(tmp, CV_8UC1, 255 / (maxval - minval), -minval);
    tmp.copyTo(output(Rect(X, 0, X, Y)));

    minMaxLoc(dy, &minval, &maxval);
    dy.convertTo(tmp, CV_8UC1, 255 / (maxval - minval), -minval);
    tmp.copyTo(output(Rect(2*X, 0, X, Y)));

    namedWindow("Img/Dx/Dy", WINDOW_AUTOSIZE);
    imshow("Img/Dx/Dy", output);
    waitKey(0);

    return 1;
}

And my suspect is eventually a misuse of the method convertTo, but I can't figure out why.

Thank you.